1. Field of the Invention
This invention relates to seamed press fabric and has particular reference to a seamed press fabric modified to reduce or substantially eliminate marking of a paper sheet by the fabric during production of the paper.
2. Description of the Related Art
Conventionally, papermaking machines have three basic sections: the forming section, the wet press section, and the drying section. In the forming section, paper pulp, which is almost liquid, is introduced onto a permeable forming fabric. The forming fabric transports the aqueous paper pulp across suction boxes thereby forming an aqueous paper web or sheet having a concentration of approximately 20% dry matter, essentially cellulose or similar fibers.
The aqueous paper web is then transported on fabrics known as wet felts, between press rolls in the wet press section of the papermaking machinery. Such a felt generally comprises, an open-mesh base fabric having at least one batt needled thereto. The pressure of the press rolls causes the water to leave the paper web; the water runs through the interstices of the wet felts. Wet felts are designed such that marking and crushing of the aqueous paper web is avoided during the dewatering process.
The paper web exits the wet section of the papermaking equipment as a sheet of approximately 45% dry matter. Thereafter, the paper sheet is carried on fabrics, known as dryer felts, through the drying section of the papermaking equipment where more water is removed through evaporation as the paper is transported around heated rollers.
In the dryer section of the papermaking equipment, the paper sheet is more resistant to marking. Conventional techniques for seaming papermaker's fabrics for the dryer section of the papermaking machine have proven satisfactory.
In contrast with the dryer section, the aqueous paper web is much softer as it passes through the wet section of the papermaking equipment. Accordingly, the risk of marking the paper is much greater. Any variation in the thickness, composition, or surface texture of the papermaker's fabric may cause marking of the aqueous web as it passes through the press rollers.
It will be appreciated that difficulty arises in the jointing of press felts since either the felts are made in a tubular fashion, or felts are made in longitudinal lengths and which need to be joined. Hitherto, only felts which are woven as endless felts have been acceptable since this tends to overcome the substantial discontinuity occasioned by other types of press felt joints. As an alternative to endless felts, loop seams in press felts have been proposed, for example, in U.S. Pat. Nos. 2,883,734 and 2,907,093, but because of the nature of the loops and seam area, it is almost inevitable that, marking of the paper web is possible as it passes a nip. With woven, single layer felts, there is little protection between the seam loops and a paper web.
Straight joints have also been proposed by securing butt edges with a backing sheet, but again this produces a hardness in the nip which causes the marking of the paper.
Seamed press felts are gaining popularity in the paper industry because they allow for easier installation and reduce paper machine down time. However, their utility has been limited due to the tendency of flap wear and sheet marking.
There is a need in the art to provide a seamed press felt that has minimum, if not an absence, of paper marking.